<span><span>The sum of a squared and b is given by

</span>7 times the sum of a squared and b is given by
</span><span>
while 4 times the sum of a squared and b is given by
</span>

Therefore, <span>7 times the sum of a squared and b minus 4 times the sum of a squared and b is given by

</span>
The two black circles represents "The selling prices that produces no profit." If the selling is 10, the profit is 0. If the selling price is 60, the profit is 0.
The selling price that provides the maximum profit is represented by the red dot. It is between the selling price of 30 and 40. Its maximum profit is equal to 12,000.
I’m not sure if it’s correct but I think you have to find the slope in order to get your answer